Plate-like ceramic heater

ABSTRACT

A heater including an electron-conductive pattern designed to generate heat, which has a partially and/or entirely stabilized ZrO 2  -base substrate, an Al 2  O 3  -base coating layer disposed on the surface of the substrate, and an electron-conductive pattern to generate heat disposed on the coating layer.

This application is a continuation of U.S. applicatin Ser. No. 805,808, filed Dec. 6, 1985, new abandoned.

FIELD OF THE INVENTION

The present invention relates to a plate-like ceramic heater and, more particularly, to means for improving the durability of a plate-like ceramic heater in which a ceramic substrate is provided thereon with an electron-conductive pattern for the purpose of generating heat.

BACKGROUND

In the prior art, there has been produced a heater having on a substrate composed mainly of Al₂ O₃ an electron-conductive pattern desired to generate heat. However, when current (direct current) is continued to be applied through the heater, blackening or peeling-off occurs in the vicinity of the cathode terminal. There occur an increase in the resistance and hence partial heat generation, which may result in deterioration of the durability of the heater.

SUMMARY OF THE INVENTION

Although still unclarified, we think that the reason for such blackening is attributable to the reduction of A₂ O₃ or impurities therein, and to the catalytic action upon the reduction reaction of Pt in the pattern diffusing into the substrate.

On the other hand, changing of the substrate material from the Al₂ O₃ -base, material to a ZrO₂ -base material serves to prevent blackening of the cathode terminal portion due to the application of current and to decrease the power required or heating an object, thereby extending the durable life of the heater to an extreme degree. This is because (1) the ZrO₂ -base material is oxygen-conductive, and (2) the ZrO₂ -base substrate dissipates less heat since the ZrO₂ -base material has a lower thermal conductivity than the Al₂ O₃ -base material. However, the electric resistance of the ZrO₂ -base material becomes very small at elevated temperatures, so that the anode and cathode terminal portions of the electron-conductive pattern have to be insulated. For that reason, these has been a demand for improving the insulating properties at elevated temperatures of the ZrO₂ -base substrate without causing deterioration of the durable life of the heater at the time of the application of current.

An object of the present invention is to eliminate the problems in the prior art referred to in the Background of the Invention. It has been found by the present inventors that this object is achieved by providing a coating layer of A₂ O₃ having a suitable thickness on the entire portion or at least an electron-conducive pattern portion of the surface of a partially and/or entirely stabilized ZrO₂ -base substrate, and further providing on said coating layer an electron-conductive pattern to generate heat.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are both illustrative of the structural examples of the heaters according to the embodiments of the present invention;

FIG. 1 showing an embodiment wherein an embodiment wherein an A₂ O₃ layer is applied to the heat-generating pattern portion alone, and

FIG. 2 showing an embodiment wherein an A₂ O₃ layer is applied on the entire surface of the ZrO₂ -base substrate.

FIG. 3 is a graphic representation showing that the mechanical strength of the ZrO₂ -base substrate is enhanced by the coating of Al₂ O₃.

DETAILED DESCRIPTION OF THE INVENTION

A dense A₂ O₃ layer is provided on the entire surface, or only on the part of the surface whereon the electron-conductive pattern is to be disposed, of the ZrO₂ -base substrate, whereby it is possible to prevent the current from escaping due to the increased conductivity of ZrO₂ at elevated temperatures. However, too thick an A₂ O₃ layer lessens the effect of the ZrO₂ -base substrate, while too thin an A₂ O₃ layer causes deterioration of the insulation of the heater so that an inadequate result is obtained. Thus, the A₂ O₃ layer according to the present invention should have a thickness of preferably 20 to 70 microns, most preferably 30 to 50 microns.

The raw material for the Al₂ O₃ -base coating layer according to the present invention contains A₂ O₃ having a purity of no lower than 90%, and may contain SiO₂, MgO, CaO, ZrO₂, etc. in addition thereto. In particular, the addition of a slight amount of ZrO₂ serves to improve the integrality (or binding force) of that layer with respect to the ZrO₂ -base substrate and, hence, reduce the sintering shrinkage modulus of that layer.

The ZrO₂ -base substrate used is formed of sintered bodies of partially stabilized or entirely stabilized ZrO₂, in which Y₂ O₃, CaO, MgO, etc. are added to ZrO₂. The electron-conductive pattern may be obtained by forming a paste composed mainly of Pt, Rh, W, Mo or a mixture thereof (which may include some amounts of oxides) on the Al₂ O₃ -base coating layer by the known techniques such as screen printing, etc., followed by heating. How to provide the electron-conductive pattern per se is well known in the art, so a more detailed description is omitted from this application as unnecessary.

The heaters of the present invention usually comprise a basic structure composed of the ZrO₂ -base substrate 4, Al₂ O₃ -base coating layer 3 and the electron-conductive pattern, i.e., heat-generating pattern 2 (or a terminal portion 6), said basic structure being sandwiched between two outer protective layers (usually of, e.g., Al₂ O₃), as indicated in FIGS. 1 and 2. An additional outer protective layer 1, e.g., an outer alumina coat layer may be provided on the outer surface of the basic structure to provide improvements in durability and prevent warpage, etc. When an additional alumina coat layer is applied on one side of the basic structure, the application of a similar alumina coat layer 5 on the other side is useful for preventing warpage. However, it is to be understood that the embodiments of the present invention are not limited to those illustrated.

It is also to be noted that, in the production of the heaters of the present invention, the structural parts may independently be sintered for assembling, but it is preferred that, after lamination, all the layers are simultaneously sintered to improve the integrality therebetween.

Preferably, the Al₂ O₃ -base material used in the present invention has a smaller sintering shrinkage modulus than the ZrO₂ -base substrate since, in the simultaneous sintering, the A₂ O₃ layer is densified owing to a contraction difference relative to the ZrO₂ -base substrate material. If the ratio of the sintering shrinkage moduli of the ZrO₂ base substrate to the A₂ O₃ layer is selected from a range of 1.01:1 to 1.08:1, then both layers contract integrally during simultaneous sintering. In consequence, not only does densification of the A₂ O₃ take place, but a compression stress is also produced in the ZrO₂ -base substrate, resulting in further increases in the the mechanical strength thereof (see FIG. 3). More marked results are obtained, especially when the thickness of the A₂ O₃ coating layer is 1/100 to 20/100 relative to the thickness of the ZrO₂ -base substrate.

According to the present invention, it is possible to improve the insulating properties of ZrO₂ substrate heaters without substantial detriment to the durability and current efficiency thereof. It is further possible to enhance considerably the mechanical strength of the heaters.

In the following, the present invention will be explained with reference to the examples.

EXAMPLES

(1) 94 mol % ZrO₂ (with the mean particle size being 0.8 microns) and 6 mol % Y₂ O₃ (with the mean particle size being 0.3 microns) were wet-mixed together for 25 hours. To avoid incorporation of impurities, ZrO₂ balls were used for mixing.

(2) After drying, the resulting mixture was passed through a 60-mesh sieve, and was sintered at 1350° C. for 2 hours.

(3) With the balls used in step (1), the sintered product was pulverized for 50 hours into powders, 80 % or more of which had a grain size of 2.5 microns.

(4) After drying, the powders were mixed together for 10 hours, using as the solvent toluene, methyl ethyl ketone, etc.

(5) Thereafter, resin was mixed to prepare a sheet-like sample of 4.2 mm in green length, 4.8 mm in green width and 0.8 mm in green thickness by the doctor blade technique.

(6) Pt black 2: Pt sponge 1 were formulated into a paste with butyl carbidol etc. as the material for the electron-conductive pattern.

(7) Next, 92 wt % Al₂ O₃, 3 wt % ZrO₂ and 3 wt % SiO₂ (and MgO, CaO) were formulated into a paste with butylcarbidol etc.

(8) The paste obtained in (7) was screen-printed on the sheet obtained in (5) into a thickness of about 50 microns. In Example 1 of Table 1, screen printing was applied to only the surface portion where the electron-conductive pattern portion is to be disposed, and in Example 2, screen printing was applied to the entire surface of the sheet.

(9) Thereafter, the Pt paste obtained in (6) was screen-printed into a thickness of about 30 microns to form a heat-gererating pattern 2 and a terminal pattern 6.

(10) Thereafter, the A₂ O₃ paste obtained in (7) was screen-printed over the entire surface into a thickness of about 50 microns.

(11) After removal of the resin at 250° C. for 12 hours, sintering was carried out at 1515° C. for 4 hours.

(12) An A₂ O₃ substrate of a shape similar to that of the examples was prepared, using as the raw material the alumina paste of (7). That substrate was coated with the Pt paste of (6), on which an A₂ O₃ coat of 50 microns in thickness was applied to prepare an A₂ O₃ substrate heater for the purpose of comparison.

(13) Current durability testing by applying a direct current of 17 V was carried out with the plate-like heaters prepared in the foregoing. The results are set forth in Table 1.

(14) At the initial stage of testing, direct current was passed at 14 V through each heater to measure the temperature thereof by means of a CA thermocouple spaced 1 mm apart from the heater surface. The temperature was about 700° C. for the heater of Example 1 and about 710° C. for the heater of Example 2. However, the heater for the comparison example showed 670° C.

                                      TABLE 1                                      __________________________________________________________________________     (Resistance Values: measured at room temperature)                              Results                                                                              Initial                                                                              after   after   after      after                                   Sample                                                                               Resistance                                                                           50 hours                                                                               100 hours                                                                              150 hours  200 hours                               __________________________________________________________________________     *1    3.8Ω                                                                           no change                                                                              no change                                                                              no color change                                                                           no color change                         Example 1                   Resistance 4.1Ω                                                                     Resistance 4.3Ω                   *2    3.8Ω                                                                           no change                                                                              no change                                                                              no color change                                                                           no color change                         Example 2                   Resistance 4.1Ω                                                                     Resistance 4.2Ω                   *3    3.9Ω                                                                           Coat portion                                                                           peeling-off of                                                                         Three of five                                      Example 3   becomes black                                                                          coat portion                                                                           samples disconnected                                           Resistance 4.2Ω                                                                  Resistance 4.5Ω                                      __________________________________________________________________________      *1: Al.sub.2 O.sub.3 was applied to only the portion beneath the               electronconductive pattern portion.                                            *2: Al.sub.2 O.sub.3 was applied on the entire surface of the ZrO.sub.2        substrate.                                                                     *3: Al.sub.2 O.sub.3 substrate heater                                     

What is claimed is:
 1. A heater, comprising a partially and/or entirely stabilized ZrO₂ -base substrate, an Al₂ O₃ -base insulating layer disposed on a surface of the substrate and having insulating properties at high temperature, and an electron-conductive pattern to generate heat disposed on said Al₂ O₃ -base layer, said substrate and insulating layer having been formed by simultaneous sintering, and wherein the A₂ O₃ layer has a thickness of at least 20 microns.
 2. A heater as defined in claim 1, wherein the sintering shrinkage modulus of the Al₂ O₃ -base insulating layer is smaller than that of the ZrO₂ -base material forming the substrate.
 3. A heater as defined in claim 2, wherein the ratio of the sintering shrinkage modulus of said ZrO₂ -base layer to the sintering shrinkage modulus of the Al₂ O₃ -base layer is 1.01 : 1 to 1.08:1.
 4. A heater as defined in claim 1, wherein the Al₂ O₃ -base insulating layer has a thickness of 1/100 to 20/100 relative to the ZrO₂ -base substrate.
 5. A heater as defined in claim 1, wherein further comprises at least one protective layer covering the electron-conductive pattern.
 6. A heater as claimed in claim 1, wherein the Al₂ O₃ -base layer is formed directly on the ZrO₂ -base substrate, and the A₂ O₃ layer is in substantially continuous contact with the ZrO₂ -base substrate.
 7. A heater, comprising a partially and/or entirely stabilized ZrO₂ -base substrate, an Al₂ O₃ -base layer disposed on a surface of the substrate and having insulating properties at high temperature, and an electron-conductive pattern to generate heat disposed on said Al₂ O₃ -base layer, and wherein the A₂ O₃ layer has a thickness of at least 20 microns.
 8. A heater as defined in claim 7, wherein the sintering shrinkage modulus of said Al₂ O₃ -base layer is smaller than that of ZrO₂ -base material forming said substrate.
 9. A heater as defined in claim 8, wherein the ratio of the sintering shrinkage modulus of said ZrO₂ -base layer to the sintering shrinkage modulus of the Al₂ O₃ -base layer is 1.01:1 to 1.08:1.
 10. A heater as defined in claim 7, wherein further comprises at least one protective layer covering the electron-conductive pattern.
 11. A heater as defined in claim 7, wherein said Al₂ O₃ -base layer has a thickness of 1/100 to 20/100 relative to the ZrO₂ -base substrate.
 12. A heater as claimed in claim 7, wherein the Al₂ O₃ -base layer is formed directly on the ZrO₂ -base substrate, and the A₂ O₃ layer is in substantially continuous contact with the ZrO₂ -base substrate. 